Friction stir welding (FSW) is a relatively new welding process for joining together parts of materials such as metals, plastics, and other materials that will soften and commingle under applied frictional heat to become integrally connected. A detailed description of the FSW apparatus and process may be found in Patent Publications WO 93/10935 and WO 95/26254; and U.S. Pat. No. 5,460,317, all of which are herein fully incorporated by reference. One useful apparatus for FSW is shown in FIGS. 1A and 1B. As shown, two parts, exemplified by plates 10a' and 10b' on a backing plate 12', are aligned so that edges of the plates to be welded together are held in direct contact. An FSW tool W' has a shoulder 14' at its distal end, and a nonconsumable welding probe 16' extending downward centrally from the shoulder. As the rotating tool W is brought into contact with the interface between plates 10a' and 10b', the rotating probe 16' is forced into contact with the material of both plates, as shown. The rotation of the probe in the material and rubbing of the shoulder against the upper surface of the material produce a large amount of frictional heating of both the welding tool and the plate interface. This heat softens the material of the plates in the vicinity of the rotating probe and shoulder, causing commingling of material, which, upon hardening, forms a weld. The tool is moved longitudinally along the interface between plates 10a' and 10b', thereby forming an elongate weld along the interface between the plates. The welding tool's shoulder 14' prevents softened material from the plates from escaping upward, and forces the material into the weld joint. When the weld is completed, the welding tool is retracted.
When FSW is used to weld together cylindrical or domelike workpieces, for example, when joining tank elements, a welding apparatus supporting the workpieces being joined must react to and oppose significant forces. FIG. 2A is a typical force diagram illustrating various forces that are created while welding flat workpieces together. Flat workpieces, collectively represented by 20, traverse in the direction of arrows shown, while an FSW tool W remains stationary. Arrow 22 represents a force applied to a point S beneath a rear surface of workpieces 20 by a probe 16 of the tool W, and arrow 24 represents a force applied to the point S by the transverse movement of workpieces 20. Arrow 26 is the minimum force required to oppose the combined forces represented by arrows 22 and 24. FIG. 2B, on the other hand, is a force diagram illustrating the forces created while welding cylindrical workpieces 20' together. Cylindrical workpieces 20' are rotated with respect to a probe 16' in the direction of arrows shown. As before, arrow 22' represents a force applied to a point T beneath a rear surface of workpieces 20' by the probe 16', and arrow 24' represents a force applied to the point T by the rotational movement of workpieces 20'. Finally, arrow 26' is the minimum force required to oppose the combined forces represented by arrows 22' and 24'. As better understood by comparing arrows 24 and 24' in FIGS. 2A and 2B, respectively, the rotary force encountered when forming a circumferential weld is significant. For example, the joining of 10 ft diameter cylindrical workpieces of 1/4 inch thick Al--Li will typically encounter a radial force corresponding with arrow 24' of approximately 8,000 ft.multidot.lb. Reasonable design margins would then call for a welding apparatus capable of reacting the resultant force 26' and driving the rotating probe 16' into the material to be joined with at least two times the radial force, e.g., 20,000 ft.multidot.lb.
In the past, an expensive rigid ring has been used to react to the significant forces associated with friction stir welding a circumferential joint. The rigid ring is placed inside the cylindrical or domelike members to be welded together along a circumferential weld joint. A friction stir welding tool is then applied along the joint from outside. Upon completion of the welding process, the ring is disassembled and removed from inside the members. Since each ring is configured for a specific diameter, different rings, each with a specific diameter, are required for joining circumferential welds of different diameters. This significantly increases tool inventory costs.
Furthermore, the ring method as described above tends to create an offset between the materials being joined. As a friction stir welding tool is rotated and applied on the materials to be joined, usually at a slight angle, it produces forces that tend to lift the materials being joined off the ring, causing the weld offset. The weld offset also occurs in the formation of a straight longitudinal weld along flat workpieces, as illustrated in FIGS. 1A and 1B.
A need exists for an apparatus and method for opposing the significant forces associated with the friction stir welding together of cylindrical or domelike members. Preferably, such apparatus and method are capable of forming circumferential welds of variable diameters. Furthermore, a need exists for an apparatus and method for eliminating weld offset in forming a circumferential weld, as well as in forming a straight longitudinal weld.